The present invention relates to a method for improving the resolution of an A/D converter, and an A/D converter system having an S/H device (S/H: sample/hold) and an A/D converter.
A/D converters (analog/digital converters) are the interface between analog and digital circuits and are used in a large number of applications, e.g., in microcontrollers and control units.
Known A/D converters, such as flash or pipeline converters, have a predetermined resolution, such as 8, 10, or 16-bit, for example, with which an analog value is converted to a binary value.
A typical A/D converter known from the related art is shown in FIG. 1 with reference numerals 4 and 5. FIG. 1 shows an A/D converter system with which an analog input signal 1 (e.g., an analog measured value) is converted to a digital output signal 7 (e.g., a digital 10-bit word). Analog input value 1 is first sampled by an S/H device 4, and sampled values 5 are each converted to a digital output signal 7 using an A/D converter 6.
The resolution is determined by the LSB (least significant bit) of A/D converter 6. A higher resolution can be attained only by using an A/D converter 6 having a higher resolution (e.g., 16-bit instead of 10-bit). To attain this, however, a substantially more complex A/D converter 6 having a substantially greater number of comparators is required, which would be correspondingly more expensive.
An object of the present invention, therefore, is to enhance the resolution of an existing A/D converter in a simple and cost-effective fashion.
According to the present invention, an auxiliary signal is superposed on an analog signal to be converted, to sample the superposed total signal using an S/H device, to convert the sampled values to digital output values using an A/D converter, and, based on a plurality of output values, to determine a binary output value having higher resolution. By superposing the auxiliary signal on the analog input signal, digital output values are produced that may differ from each other by at least one bit. Based on the number of output values having a higher bit value or the number of output values having a lower bit value, it is possible to determine, for example, in which bit resolution range the analog input signal is actually located. The resolution may thus be enhanced.
According to a first embodiment of the present invention, the superposed signal is sampled multiple times, and the sampled values are each converted to digital output values, the mean of the digital output values then being calculated. When n sampled values (e.g., n=8) are evaluated, it is possible to attain an additional resolution of √{square root over (n+1)} bit (3-bit). Since the superposed signal is to be sampled, however, the high-resolution output value is not available until after n sampling steps (e.g., n=8). The digital output values having the resolution predetermined by the A/D converter are still available in real time as before, however.
A high-resolution output value 10 may also be determined from the ratio of the number of output values 7 having a higher bit value, or the number of output values 7 having a lower bit value to the total number of output values 7 that were input.
The superposed signal is preferably sampled at a frequency that enables the recording of sampled values on the positive and the negative amplitude of the superposed signal. The sampling frequency is preferably selected so that a beat is produced with regard to the sampled values on the positive amplitude as well as the sampled values on the negative amplitude of the superposed signal.
The auxiliary signal preferably has a peak-to-peak amplitude that is greater than or equal to the resolution of the least significant bit (LSB) of the digital output values. If the peak-to-peak amplitude of the auxiliary signal is at least as great as the resolution of the LSB, the noise (e.g., thermal noise) of the A/D converter may be reduced. Furthermore, a portion of the linearity error of the A/D converter may be compensated.
The auxiliary signal added to the input signal is preferably a periodic signal, such as a sine or square-wave signal.
The sampling frequency of the S/H device is preferably approximately two-fold greater than the frequency of the auxiliary signal.
Accordingly, an A/D converter system for improving the resolution of an A/D converter includes a device for superposing an auxiliary signal on an analog signal, a superposed signal being produced, an S/H device for sampling the superposed signal, and an A/D converter that converts the sampled values to digital output values. The A/D converter system further includes a processing unit that determines, based on the digital output values, a digital output value having higher resolution, e.g., by averaging.
The device for superposing the auxiliary signal may include a capacitor or a current source, for example.